Substrate with a photocatalytic coating

ABSTRACT

The present invention is directed to a process for obtaining a substrate provided with a coating having photocatalytic properties, wherein the coating includes crystallized particles of an oxide of a metal A having photocatalytic properties. The crystallized particles are incorporated into the coating using a mineral binder comprising at least one oxide of a metal B also having photocatalytic properties in the crystallized state. The coating optionally includes at least oxide of a metal M devoid of photocatalytic properties and/or at least one silicon compound of the silicon oxide SiO 2  type. The coating is deposited from liquid-phase dispersions containing the crystallized particles of the oxide of metal A and at least one precursor compound for the oxide of metal B of the binder and optionally a precursor compound for the oxide of metal M and for the Si compound, in a relative proportion A/(B+M+Si) by weight of the metals and Si ranging between 60/40 and 40/60. The present invention is also directed to substrates containing a photocatalytic coating and to liquid-phase dispersions which are used in the preparation of the photocatalytic coatings.

This is a continuation of the U.S. national phase designation of PCTapplication no. PCT/FR99/00511, filed Mar. 5, 1999.

FIELD OF THE INVENTION

The present invention relates to substrates provided with aphotocatalytic coating and the process for obtaining such a coating andits various applications. In particular, the present invention relatesto coatings comprising semiconductor materials based on a metal oxide,which upon irradiation at a suitable wavelength, is capable ofinitiating radical reactions.

BACKGROUND OF THE INVENTION

Coatings confer novel functionalities on the materials to which they areapplied. These functionalities include antisoiling, fungicidal, andbactericidal properties and are optionally combined with uses such asrepelling water, providing an anti-fogging layer, and opticallymodifying a substrate.

A wide variety of materials may be used as coating substrates. Examplesof these materials are those used in vehicles or buildings, such asglazing products, walling, cladding, roofing and flooring materials suchas tiles, slates, slabs and pavings. Practically any material used inthe construction industry can be used as a substrate. These materialsmay be made of glass, metal, glass-ceramic, ceramic, cement brick, wood,stone, as well as materials reconstituted from natural materials,plastic, or fibrous materials.

Transparent substrates are typically used as glazings. These transparentsubstrates include glass and flexible or rigid plastic such as thosemade of polyester or acrylate. A particular example of an acrylatesubstrate is polymethyl methacrylate (PMMA).

Substrates may be classified according to their porosity. Thus,substrates may be characterized as porous, non-porous, or slightlyporous. Substrates may also be regarded as a single material such as aglass substrate, or as a composite material such as a walling materialwhich is provided with a coating of the wall-render type.

Coatings containing crystallized anatase TiO₂ which have photocatalyticproperties have been disclosed in patent applications WO 97/10186 and WO97/10185. The coatings described in those references are obtained fromthe thermal decomposition of suitable organometallic precursors and/orfrom precrystallized TiO₂ particles embedded in a mineral or organicbinder.

SUMMARY OF THE INVENTION

The present invention is directed to substrates comprising coatingshaving photocatalytic properties. The coatings may additionally possessanti-soiling, bactericidal, fungicidal, anti-fogging, or water-repellantproperties. They may be used also to modify the optical properties of asubstrate. The coatings of the present invention comprise at least onecomponent having photocatalytic properties and a binder which is alsophotocatalytic. Optional components may be incorporated into thecoating. The substrates of the present invention comprise various typesof materials such as glass, plastic, textile, composite materials, andwalling, cladding, or roofing materials.

The present invention is also directed to a process for preparingsubstrates which comprises a coating as described above. Further, thepresent invention relates to liquid-phase dispersions used in thepreparation of the coatings for the substrates of the present invention.

The coatings applied to the substrates of the present invention exhibitboth a satisfactory level of photocatalytic activity and an enduringcoating durability. Simultaneous optimization of the photocatalyticactivity and coating durability, properties which were previouslybelieved to be incompatible, was achieved mainly by optimizing therelative proportions of the components comprising the coatings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1: A schematic of the structure of the photocatalytic coating.

FIG. 2: A photograph of the surface of a photocatalytic coating obtainedthrough a scanning electron microscopy (SEM).

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

An object of the invention is to enhance the properties of coatings byextending their photocatalytic performance after repeated exposure toaging conditions encountered in various types of applications. Inparticular, the photocatalytic performance of the coatings are improvedpartly by enhancing their mechanical or chemical durability.

The process of the present invention comprises obtaining a substratehaving a surface at least part of which is coated with a photocatalyticcoating. The photocatalytic coating comprises crystallized particles ofan oxide of a metal A having photocatalytic properties. Thesecrystallized particles are incorporated into the coating using a mineralbinder comprising at least one oxide of a metal B also havingphotocatalytic properties in the crystallized state. This binder mayoptionally comprise at least one oxide of a metal M devoid ofphotocatalytic properties or at least one silicon (Si) oxide compound.

The process of the present invention comprises depositing the coatingfrom one or more liquid-phase dispersions comprising: (a) crystallizedparticles of the oxide of metal A; (b) at least one precursor compoundfor the oxide of metal B which forms at least part of the mineral binderand (c) optionally a precursor compound for the oxide of metal M and/orfor the Si compound. Preferably, the ratio of the weight of metal A tothe weights of the other components such as B, M, or Si ranges between60/40 and 40/60.

Preferably, the coating deposition/treatment conditions are chosen suchthat the mineral binder, particularly the oxide of B which forms atleast part of the binder, is at least partially crystallized in thefinal coating. Preferably, the oxides of metals A and B are chosen fromat least one of the following: titanium oxide, zinc oxide, tin oxide,and tungsten oxide. A particularly preferred oxide of A or B is atitanium oxide, the anatase crystal form of which is highlyphotocatalytic. Examples of the metal M oxides which are devoid ofintrinsic photocatalytic properties are aluminum oxide or zirconiumoxide.

The present invention has allowed the simultaneous optimization of twocoating properties, photocatalytic performance and coating durability,which previously appeared to be incompatible. More specifically, thepresent invention has succeeded in extending the photocatalyticproperties of the coating over time. Because the catalytic effect of thecoating might be mostly due to the incorporated particles which hadalready crystallized and had been catalytically active from the verybeginning, the temptation is to maximize the amount of particles in thecoating. Surprisingly, both excessively high and excessively low amountsof particles have turned out to be unsuitable for achieving both thedesired photocatalytic properties and coating durability. At least twofactors have made it difficult to find an optimal ratio of the coating'scomponents: (a) the amount and type of particles of the oxide of metal Aprobably influence the morphology of the binder, and (b) the change inthe photocatalytic character of the coating and in its retention overtime is not a linear function of a parameter such as (a).

The process of the present invention demonstrates that it is possible toselect a ratio such as A/(B+M+Si) which allows both a satisfactory levelof photocatalytic activity and an enduring photocatalytic activity. Thereasons for this are not completely understood. It is possible that themineral binder also contributes to the coating's photocatalyticactivity. The coatings of the present invention has the additionaladvantage of possessing excellent optical properties such as high lighttransmission and very low haze level.

The precursors for the oxide of metal B, and those for the optionaloxide or oxides of metal M, are preferably organometallic compoundscapable of decomposing into an oxide under suitable treatment such as aheat treatment. A silicon alkoxide (silane) may be used as a precursorfor the Si compound, particularly for the SiO₂.

Preferably, the process of the present invention uses crystallizedparticles of the oxide of A. Particularly preferred is TiO₂ crystallizedpredominantly in the anatase form. Preferably, the particles are in theform of agglomerates of crystallites preferably having a mean size ofapproximately 5 nm to 80 nm. The crystallites preferably have a meansize of between about 5 nm to 20 nm, more preferably between 5 nm to 10nm, in a liquid phase dispersion, preferably in an aqueous-basedcolloidal suspension or in a dispersion having at least one organicsolvent. The particle sizes correspond approximately to the diameters ofthe agglomerates and crystallites in question, although the particles oragglomerates may assumes shapes other than spherical such as lenticularor rods.

Rather than speak of agglomerates of crystallites, one may instead referto the agglomerates as particles and to the crystallites as crystallinecoherence domains. To a first approximation, the agglomerates areassumed to have undergone little or no structural or dimensional changein the final coating. In fact when the process for preparing thephotocatalytic coating involves heat treatment, the particlesstructurally change, with the crystallites' size increasing appreciably.For example, from an initial approximate size of 5 nm to 10 nm, the TiO₂crystallites' size changes to about 10 nm to 20 nm in the final coating.The crystallites' size has thus approximately doubled. A detaileddescription of these particles are found in WO 97/10185, WO 98/23549, orFR-2,681,534.

Preferably, a precursor organometallic compound for the oxide of metalB, and optionally for the oxide of metal M, is chosen from the groupconsisting of tetraalkoxides of formula X(OR)₄, wherein X refers to ametal and R is a linear or branched alkyl-type carbon-containingradical. The R's may be identical or different and preferably comprisesfrom 1 to 6 carbon atoms. Examples of these precursors are titaniumtetrabutoxide or titanium tetraisopropoxide. The precursors may also bechosen from trialkoxides of the formula MR′(OR)₃, where R and R′ areradicals which are identical to or different from those in theabovementioned tetraalkoxides. The precursors can also be halides,preferably titanium chlorides.

Because the above precursors are highly hydrolysable and reactive, theyare preferably dissolved with at least one chelating or stabilizingagent. Examples of these chelating or stabilizing agents are β-diketonessuch as acetylacetone (2,4-pentanedione), benzoylacetone(1-phenyl-1,3-butanedione), and diisopropylacetylacetone. One may alsouse acetic acid, diethanolamine, or glycols such as ethylene glycol ortetraoctylene glycol as chelating or stabilizing agent. The precursorconcentration in the solution for a given solids content is adjusted byusing one or more organic solvents as diluent.

In one embodiment of the present invention, the mineral binder of thecoating comprises only an oxide of metal B. Thus, the A/(B+M+Si) ratiomentioned above reduces to A/B. In another embodiment of the presentinvention, the mineral binder comprises an oxide of metal B of the TiO₂type, and a silicon compound of the SiO₂ type, the A/(B+M+Si) ratio thusbecoming A/(B+Si).

A particularly preferred embodiment of the process of the presentinvention comprises depositing the coating from two dispersions, onecomprising at least one precursor and another comprising the particles,the two dispersions being premixed into a single dispersion beforespraying onto the substrate. Alternatively, the substrate is immersed inthis combined dispersions, although one may deposit a coating usingseveral separate dispersions without premixing them.

One type of deposition technique is called hot deposition. In thistechnique, the substrate, during the dispersion/substrate contact, is ata temperature high enough to allow thermal decomposition of theprecursor(s). This is a liquid-phase pyrolysis type technique.

Another technique is cold deposition. In this technique, the substrate,during the dispersion/substrate contact, is at room temperature or atthe very least at a temperature too low to cause decomposition of theprecursor(s). This is a sol-gel type technique. This method ofdeposition includes dipping, cell-coating, laminar-coating, orspray-coating.

A heat treatment following contact between the dispersion and substrateis necessary in a cold deposition techniques. The heat treatment curesthe coating and ensures that the precursors have completely decomposed.But heat treatment is also advantageous in a hot deposition techniquesbecause it may improve the cohesion of the coating and induce at leastthe partial crystallization of the binder arising from the decompositionof the precursor(s). Preferably, the heat treatment is carried out attemperatures not lower than 400° C., more preferably above 450° C., andmost preferably in the region of 500° C. to 550° C. if the substrate isable to withstand these temperatures such as when the substrates have aglass, ceramic or glass-ceramic matrix.

The present invention is also directed to a substrate provided on atleast part of its surface with a photocatalytic coating. The coatingpreferably incorporates crystallized particles of an oxide of a metal Awith photocatalytic properties. Preferably, the crystallized particlesare incorporated using an at least partially crystallized mineral bindercomprising an oxide of a metal B also having photocatalytic propertiesin the crystallized state. Preferably, the substrate is obtainedaccording to the process described above.

The substrate of the present invention is characterized by high porositypreferably greater than 40%, and more preferably between 45% and 65%.This porosity can be calculated indirectly from a measurement of therefractive index of the layer and then comparison with what the indexwould be if the material were fully dense. Because the index measurementalso takes into account, at least partly, the degree of surfaceroughness of the layer, this indirect method also provides a reliablemeasure of the porosity and surface morphology of the layer. Otherindirect methods may be used, especially those which involve measuringthe weight of the coating deposited per unit area of the substraterelative to a given coating thickness.

A high porosity substrate has many advantages. First, it makes itpossible to decrease the refractive index of the material and thus varyits optical appearance. In a TiO₂-based coating comprising TiO₂particles (crystallized predominantly as anatase), a TiO₂-based binder,and an optional Si oxide, decreasing the substrate's index below 2,preferably about 1.4 to 1.8, and more preferably about 1.7 to 1.8,allows a substrate's reflectivity to be very greatly diminished.

Also, the porosity of the coating is linked with high surface roughness.A high surface roughness in turn correlates with a highly developedsurface area of the coating which favors photocatalytic activity.

There are at least two different kinds of roughness. They are describedin patent WO 98/23549. Surface roughness gives a coating an enhanced andlasting hydrophobic property thus providing the coating a pronouncedwater-repellant and anti-fogging properties. The hydrophobicity of thecoating also promotes the removal of dirt from rainwater. Surprisingly,this high porosity does not excessively weaken the coating mechanically.

The present invention is also directed to a substrate on at least partof its surface is provided a coating having photocatalytic properties.This substrate's coating comprises crystallized particles of an oxide ofa metal A having photocatalytic properties. The crystallized particlesare incorporated using an at least partially crystallized bindercomprising at least one oxide of a metal B also having photocatalyticproperties in the crystallized state. In one embodiment of the presentinvention, the substrate further comprises at least one oxide of a metalM devoid of photocatalytic properties and/or a silicon oxide compound.Preferably, the substrate's coating is obtained according to the processdescribed above.

Preferably, the ratio given by A/(B+M+Si) ranges between 60/40 and 40/60in terms of the weight of the metals (and optionally the Si) containedin the oxides of A, B, M, or Si.

The substrate of the present invention may have a fibrous appearance ora certain porosity, as in the case of a tile or a mineral insulationwool. As used herein, providing a substrate with a photocatalyticcoating may mean that the coating is deposited on the substrate'ssurface, or alternatively that the coating may partly impregnate asubstrate to within a certain depth when the substrate is porous orfibrous. Thus, the amount of coating deposited may be expressed eitherin terms of its thickness of the coating layer on the substrate when thelatter is not porous, or by the amount of material per unit area whenthe substrate is porous.

Preferably, the coating of the invention, whether obtained by theprocess described above or whether it is of a type having thecharacteristics described above, comprises the following characteristicsand components: (a) crystallized particles with size between 5 nm and 80nm; (b) crystalline coherence domains with size between 5 nm and 20 nm;and (c) a mineral binder at least partly in the form of grains locatedaround the crystallized particles in the inter-particle interstices, thegrains having a mean size between 5 nm and 25 nm, preferably between 10nm to 20 nm. In a preferred embodiment of the present invention, thesegrains, which are approximately spherical, are only partiallycrystallized. They may also be partially crystallized on a scale toosmall to measure. Preferably, the grains encapsulate the particles andbind them together, particularly in the case where the oxides of A and Bare both based on TiO₂.

Preferably, the substrate is provided with a photocatalytic coatingwhich comprises TiO₂ particles essentially in anatase form, and amineral binder comprising partially crystallized TiO₂ and SiO₂.Preferably, the coating has an index of at most 2, more preferablybetween 1.5 and 1.9, and most preferably between 1.6 and 1.8.

In one embodiment of the present invention, at least one layer isinserted between the substrate and the photocatalytic coating. The oneor more layers may have one or more functions such as modifying thesubstrate's optical properties, acting as a barrier to prevent speciessuch as alkali metals from migrating from the substrate, as anantistatic, or as an adhesion layer. At least one layer may be based onSi compounds such as SiO₂, SiON, SiOC and Si₃N₄, or on an optionallydoped metal oxide such as F:SnO₂ or Sb:SnO₂.

The substrates provided with the coatings may comprise a transparentmaterial such as glass or plastic. The coating of the present inventionmay be applied as part of a glazing used in buildings or vehicles. Thecoatings may also be used in monitor displays such as televisionscreens, computer screens, or touch screens. The coatings may form partof a laminated or monolithic glazing (i.e. a glazing comprising a singleglass pane or a single sheet of plastic). The coating of the presentinvention may also form part of an insulating multiple glazing structurein which the coating is either on an internal or external face of theglazing. The coatings may be used in a conventional insulating glazinghaving one or more gas interlayers such as those marketed bySaint-Gobain Vitrage under the names BIVER, CLIMALIT D, CONTRATHERM,CONTRASONOR, CONTRARISC. Further, the coatings of the present inventionmay be used in glazings having one or more vacuum interlayers (referredto here as vacuum glazings) such as the one described in EP-645,516. Inthe case of vacuum glazing, it is particularly advantageous to apply thecoating on an external face of the glazing for purposes such as toprevent the formation of fog on the substrate's surface.

Another substrate to which the coatings of the present invention may beapplied are glazings used in freezers or refrigerators. In fact, manymaterials may act as substrates for the coatings of the presentinvention. These materials include ceramic, plastic, cement, and otherconstruction or building materials such as walling, cladding, or roofingmaterials. Other materials suitable for use with the coatings of thepresent invention include interior or exterior floors or walls ofdwellings such as slabs or tiling.

The coating of the present invention may be deposited on fibrousmaterials, such as a mineral wool material, which may be used forapplications such as thermal or acoustic insulation. The coating mayalso be applied to fibers of the textile-yarn type as a reinforcement orfor other applications such as filtration. Further, one may takeadvantage of the antisoiling, bactericidal, fungicidal, or antifoggingproperties of the coating where desired or needed.

The present invention is also directed to liquid-phase dispersions suchas those described above. The dispersions can be used in the preparationor manufacture of the photocatalytic coating of the present invention.Preferably, the dispersions comprise a solvent selected from one of thefollowing: water, ethylene glycol, ethanol, propylene glycol and theircombinations.

The crystalline phase of the titanium dioxide particles of thedispersions is preferably predominantly in the anatase crystal form.“Predominantly” means that the anatase content of the titanium dioxideparticles of the coating is greater than 50% by weight. Preferably, theparticles of the coating have an anatase content greater than 80%. Thecrystallinity and the nature of the crystalline phase are measured byX-ray diffraction.

The dispersions of the present invention are generally obtained bymixing a dispersion of the titanium dioxide particles with solutionscomprising at least one precursor compound and/or a silicon compound.Depending on the nature of the compounds used, one may add during themixing step additives such as cosolvents, surfactants, or stabilizers.The mixing may be improved by stirring the dispersion ultrasonically.

EXAMPLES

A first series of examples below relates to the deposition on atransparent substrate 1 of antisoiling coating 3 comprising a titaniumoxide. Substrate 1 is made of clear, flat silica-soda-lime glass 15×40cm² in area and 4 mm thick. Other types of substrate may be used. Thesubstrates used may have varying degrees of curvature.

Between coating 3 and substrate 1, there may optionally be a thin layer2 based on silicon oxycarbide (SiOC) to act a barrier to the diffusionof alkali metals (this being deleterious to the photocatalytic propertyof the coating), and/or a layer which has an optical function. The oneor more layers may be deposited by known technique such as chemicalvapor deposition (CVD). The one or more layers preferably have athickness of approximately 50 nm.

Two different cold deposition processes were used. These were depositionby dipping in cell-coating mode with a bath draining rate of about 5 to30 cm/minute, and deposition by spray-coating, more specifically coldliquid spraying. These well-known techniques are explained in detail inthe patent applications referred to above.

The coatings are deposited from a dispersion obtained by mixing twoinitial solutions/dispersions 1 and 2:

solution 1: contains the organometallic precursor for the TiO₂-basedmineral binder. This precursor is titanium isopropylate Ti(OCH(CH₃)₂)₄stabilized with acetylacetonate CH₃—CO—CH₂—CO—CH₃ in an ethanolsolution;

solution 2: this is the ethylene glycol liquid phase containing thephotocatalytic crystallized particles having the followingcharacteristics:

specific surface area of the particles: ±350 m²/g

size of the particles: ˜40 nm

size of the crystallites which constitute the particles: 7 nm

crystalline phase: more than 80% anatase.

TABLE 1 TI₍₂₎/Ti₍₁₎ e (nm) T_(L) (%) haze (%) EXAMPLE 1  0/100 20 88.50.88 EXAMPLE 2 20/80 20 to 30 89.6 0.27 EXAMPLE 3 40/60 40 to 50 89.50.76 EXAMPLE 4 50/50 40 to 60 90.2 0.50 EXAMPLE 5 60/40 60 90.5 0.61EXAMPLE 6 80/20 30 to 40 89.7 0.44 EXAMPLE 7 100/0  30 to 40 90.3 1 *theTi₍₂₎/Ti₍₁₎ ratio is as defined above (unitless). *e is the thickness ofthe coating, in nm. *T_(L) is the light transmission in %, measuredusing the D₆₅ illuminant; *the haze value, in %, is given by the ratioof the diffuse transmission to the light transmission integrated overthe entire visible range.

The composition of the dispersion obtained by mixing solution 1 withdispersion 2 is adjusted to obtain the desired Ti₍₂₎/Ti₍₁₎ ratio, whichis the ratio of the weight of titanium (2) from the particles in thedispersion 2 to the weight of titanium (1) from the precursor insolution 1. The ratio may also be expressed in terms of the weight oftitanium oxide originating from the particles to the weight of titaniumoxide coming from the metal precursor, assuming that 100% of theprecursor is converted into the oxide. The same ratio would be obtained,either way.

Examples 1 to 7 relate to deposition by dipping using a cell-coatingmode under deposition conditions similar to those used above (such as abath draining rate of 6 cm/minute) and a titanium concentration (namely3% solids content, based on the weight of the oxide) identical to thatin solution 1. After deposition, the substrates undergo a heat treatmentat around 450-500° C. for at least 30 minutes.

Example 8 was produced from a dispersion prepared as in Example 4, butwas deposited on a substrate by cold spraying using a so-called airlessspray nozzle at a pressure of 0.7 bar (0.7×10⁵ Pa). The layer obtained,after heat treatment at around 450-500° C. for at least 30 minutes, hasa thickness of approximately 35 nm to 60 nm, with a T_(L) value of88.6%, and a haze value of 0.6%.

Examples 1 to 8 above were evaluated for their photocatalytic activitybefore and after exposure to conditions simulating those that lead toaging of the coating, although in this case the aging process isaccelerated.

The photocatalytic activity is measured as follows:

1. using approximately 15 cm² of coating as test sample;

2. measuring the weight of the specimen, thickness of the substrate,T_(L) and haze;

3. spray-depositing a solution of palmitic acid (8 g of acid per 1 literof chloroform) with a glass/spray nozzle at a distance of 20 cm with thesubstrate vertical, using 3 to 4 successive passes;

4. weighing the specimen, after depositing the palmitic acid, todetermine the thickness of the deposited acid (in nanometers);

5. measuring the haze and T_(I) after deposition;

6. measuring the change in haze as a function of the UVA irradiationtime (˜30 V/m²);

7. graphically determining the time at which the haze has decreased by50%, this time being called T,_(½) (disappearance);

8. evaluating the photocatalytic activity of the coating in terms of therate of disappearance, ν, defined by ν (nm/h)=[palmitic acid thickness(nm)]/[2×t_(½)(h)].

The aging of the coatings consists in subjecting them to mechanicalabrasion under the following conditions:

specimen size: 7 cm×15 cm;

applied load: 600 grams

area of the abrasion felt: 1.5 cm²;

number of cycles, n: 200 and 500 (1 cycle=1 forward and backwardmovement of the carrier arm of the felt and load).

Table 2 below gives, for each of the examples:

(a) the rate of disappearance ν1 before abrasion;

(b) the rate of disappearance ν2 after 200 cycles;

(c) the rate of disappearance ν3 after 500 cycles.

Analyses of the results show that the coatings of Examples 3, 4, and 5had a structure probably similar to that shown schematically in FIG. 1,which depicts a glass substrate 1, a SiOC layer 2 and a coating 3. Thecoating comprises particles or crystallite aggregates 5 between whichamorphous or partly crystallized TiO₂ grains 4 agglomerate, theseagglomerates forming the mineral binder of the coating.

FIG. 2, corresponding to Example 4, is a photograph obtained by scanningelectron microscopy. The photograph provides information about theappearance of the surface of Example 4. As shown, the surface isrelatively rough, allowing the coating to have a large developed surfacearea.

TABLE 2 v1 v2 v3 (0 cycles) (200 cycles) (500 cycles) EXAMPLE 1 20 10 0EXAMPLE 2 40  5-10 0 EXAMPLE 3 35 — ≈10 EXAMPLE 4 70 10-20 ≈10 EXAMPLE 535 — ≈5 EXAMPLE 6 20 1-2 0 EXAMPLE 7 20 0 0 EXAMPLE 8 40 — 7

The coating of Example 4 has a refractive index of about 1.65. To afirst approximation, one may calculate the porosity of the coating usingthe known refractive index of the bulk TiO₂ which is 2.4. Thus, theporosity is given by (2.4−1.65)/(2.4−1)×100 which is approximately 54%.The coating of Example 4 exhibits strong hydrophobicity. After exposingit to UVA rays for 20 minutes to activate it, the coating is placed inthe dark and the contact angle Φ with respect to water is periodicallymeasured. The contact angle remained less than 10° for at least 20 daysin the dark.

The following conclusions may be drawn from the data: Examples 3, 4, 6and 8, and particularly Example 5, have Ti₍₂₎/Ti₍₁₎ ratios of 50/50.These examples possess all the properties desired in a coating, namely:

a high T_(L), a low haze and a refractive index less than 2 giving thecoating a desirable optical appearance;

satisfactory photocatalytic activity, which remains even aftermechanical abrasion, which proves the durability of the coatings.

These results show the coatings can be used advantageously in actual,real-life applications. In fact, these examples retained theirphotocatalytic activity, albeit moderate, even after 500 abrasioncycles.

The second series of examples below relates to coatings prepared usingthe same solution 1 and the same dispersion 2 above. The depositionconditions are identical to those of Examples 1 to 7, except that thebath draining rate is higher, being equal to 24 cm/minute. Anotherdifference is the substrate: it is the same glass but depositedbeforehand with a 50 nm first layer of SiOC using CVD, and thendeposited with a 450 nm second layer of fluorine-doped tin oxide F:SnO₂by powder pyrolysis. In this series of examples, the amount Q ofphotocatalytic coating is evaluated not by measuring its thickness butby measuring the amount of material per unit area of the substrate,expressed in μg/cm².

The photocatalytic activity of the examples is measured before anyabrasion test. This gives the value ν1 defined above. The durability ofthe coating is tested qualitatively by wiping with a rag: “++” meansthat the coating is very strongly resistant, “+” means that it isacceptably resistant, and “−” means that most, if not all, of thecoating has been removed after wiping with a rag.

Table 3 below gives, for Examples 9 to 13, the Ti₍₂₎/Ti₍₁₎ ratios asdefined above in Table 1, the value Q, the value ν1, and the rag-wipingtest rating:

TABLE 3 Ti₍₂₎/Ti₍₁₎ e (nm) T_(L) (%) haze (%) EXAMPLE 9  0/100 22  18 ±EXAMPLE 10 20/80 24 128 + EXAMPLE 11 40/60 23 159 + EXAMPLE 12 50/50 25231 + EXAMPLE 13 100/0  23 222 −

With an identical or almost identical amount of coating deposited,optimum performance occurs for Examples 11 and 12 which have Ti₍₂₎/Ti₍₁₎ratios of 40/60 and 50/50, respectively. This same tendency may be seenin the first series of examples. Only Example 12 exhibits aphotocatalytic activity above 200 and an acceptable durability.

A third series of examples relates to a coating using TiO₂ particlesfrom the dispersion used in all the previous examples, but with a hybridbinder combining TiO₂ with SiO₂.

The solution containing the precursors for the binder uses as:

solvent: ethanol and ethylene glycol in a 75/25 ratio by weight;

stabilizer: acetylacetonate;

TiO₂ precursor: titanium tetrabutoxide (TBT); and

SiO₂ precursor: tetraethylorthosilicate (TEOS).

The relative proportion of TBT to TEOS is adjusted so as to have aTiO₂/SiO₂ ratio of 15/85 by weight in the solution (using the assumptionthat all TPT is converted into TiO₂ and all TEOS into SiO₂).

Next, an amount of the solution is added to the particle dispersion usedin the previous examples to give the desired ratio (denoted as r) givenby Ti_(particles)/(Ti_(precursor)+Si_(precursor)). The solids content ofthe solution is 3%.

The substrate and deposition conditions are identical to those inExamples 9 to 13.

Table 4 below gives the values of the ratio r, the rate ν1, the lightreflection of the coated substrate R_(L)(%), and ΔT_(L) which refers tothe variation of T_(L) observed after 500 cycles of the abrasion testperformed as in the first series of examples. Table 4 also presents theratio r₁ which expresses the ratio of the amount of Ti to the amount ofSi in terms of the weights of their oxides:

r ₁=TiO_(2 particles)/(TiO_(2 binder)+SiO_(2 binder))

The table also shows: Q₁, the total amount of TiO₂ in the coating(particles and TiO₂ resulting from the titanium precursor) in μg/cm²;and Q₂, the calculated amount, as total weight of the coating, also inμg/cm².

TABLE 4 r r₁ vl R_(L) ΔT_(L) Q₁ Q₂ EXAMPLE 14 45.1/54.9 40/60 8 12.5 08.8 18.2 EXAMPLE 15 55.3/44.7 50/50 40 10 1 9.3 16.3 EXAMPLE 16 65/3560/40 45 14 4 10.1 15.4

By adjusting the solids content of the solution and the draining rate,coatings were repeated by fixing the ratio r at 55.3/44.7 and by varyingthe amount of coating deposited.

For these additional examples from Example 5, Table 5 lists the valuesof Q in μg/cm², the corresponding thickness e in nm, ν in nm/h, R_(L),and ΔT_(L).

The data from this third series of examples show there is an advantageto adding a SiO₂ material to the binder. Although the silicon oxide isnot involved in the photocatalysis, it makes the coating morehomogeneous and tends to enhance its durability. Although the ratio r iskey, other parameters particularly Q (which preferably ranges between 15and 45 μg/cm²) are also important in accounting both for the cost of thecoating and the impact of its thickness on the optical appearance.

TABLE 5 Q e vI R_(L) ΔT_(L) EXAMPLE 15a 17 110 42 8.5 1.5 EXAMPLE 15b 33— 88 12.1 2.5 EXAMPLE 15c 49 460 130 11 0.8

The photocatalytic activity of the particles 5 may be further improvedby doping them, by introducing dopants into the crystal lattice or bycovering the particles with dopants comprising Fe, Cu, Ru, Mo, Bi, Ta,Nb, Co, Ni, or Va as described in patent WO 97/10185. One may also add amineral binder comprising oxides which are not photocatalytic or onlyslightly photocatalytic in the crystallized state. For example, one mayadd to the dispersion a precursor for a silicon oxide such astetraethoxysilane TEOS. In certain cases, the A/(B+M+Si) ratio mentionedabove, which is optimal in the 40/60 to 60 to 40 range, may lie withinthe 35/65 to 40/60 and 65/35 to 60/40 ranges.

What is claimed is:
 1. A coated substrate prepared by a processcomprising: (a) preparing a substrate for deposition with a coating,wherein the coating comprises crystallized particles of an oxide of ametal A having photocatalytic properties, a mineral binder comprising atleast one oxide of a metal B having photocatalytic properties,optionally at least one oxide of a metal M devoid of photocatalyticproperties, and optionally at least one silicon oxide; (b) depositingthe coating onto a surface of the substrate, by depositing the coatingfrom liquid-phase dispersions containing oxides of the metals A and B,optionally the oxide of metal M and optionally the silicon oxide, in arelative proportion by weight of the metals and Si given by A/(B+M+Si),the relative proportion ranging from 60/40 to 40/60; and (c) allowingthe coating to set; to provide the coated substrate, wherein the coatingcomprises crystallized particles of the oxide of metal A havingphotocatalytic properties incorporated into the coating using an atleast partially crystallized mineral binder comprising the oxide ofmetal B also having photocatalytic properties in the crystallized state,optionally at least one oxide of a metal M devoid of photocatalyticproperties and optionally a silicon oxide, wherein the coating has aporosity of between 45% and 65%.
 2. The coated substrate of claim 1,wherein the crystallized particles have a size of about 5 nm to about 80nm.
 3. The coated substrate of claim 1, wherein the mineral binder is atleast partially in the form of grains having a size of between 5 nm to25 nm.
 4. The coated substrate of claim 1, wherein the coating comprisesa partially crystallized mineral binder comprising TiO₂ and crystallizedTiO₂ particles essentially in anatase form.
 5. The coated substrate ofclaim 1, wherein at least one layer is inserted between the substrateand the coating.
 6. The coated substrate of claim 5, wherein the atleast one layer functions as a barrier to alkali metals or has anoptical, antistatic, or adhesion function.
 7. The coated substrate ofclaim 6, wherein the at least one layer is based on a Si compound or adoped metal oxide.
 8. The coated substrate of claim 7, wherein the dopedmetal oxide is a fluorine-doped tin oxide.
 9. The coated substrate ofclaim 1, wherein the substrate comprises at least one transparentmaterial of glass, plastic, or composite materials.
 10. The coatedsubstrate of claim 1, wherein the coating comprises silicon oxide. 11.The coated substrate of claim 1, wherein the at least one oxide of metalB is selected from oxide of Ti, Zn, Sn, and W.
 12. A coated substrateprepared by a process comprising: (a) preparing a substrate fordeposition with a coating, wherein the coating comprises crystallizedparticles of an oxide of a metal A having photocatalytic properties, amineral binder comprising at least one oxide of a metal B havingphotocatalytic properties, optionally at least one oxide of a metal Mdevoid of photocatalytic properties, and optionally at least one siliconoxide; (b) depositing the coating onto a surface of the substrate, bydepositing the coating from liquid-phase dispersions containing oxidesof the metals A and B, optionally the oxide of metal M and optionallythe silicon oxide, in a relative proportion by weight of the metals andSi given by A/(B+M+Si), the relative proportion ranging from 60/40 to40/60; and (c) allowing the coating to set, to provide the coatedsubstrate, wherein the coating comprises crystallized particles of theoxide of metal A having photocatalytic properties incorporated into thecoating using an at least partially crystallized mineral bindercomprising the oxide of metal B also having photocatalytic properties inthe crystallized state, optionally at least one oxide of a metal Mdevoid of photocatalytic properties and optionally a silicon oxide,wherein the coating has a refractive index of between about 1.5 and 2.13. The coated substrate of claim 12, wherein the coating comprisessilicon oxide.
 14. The coated substrate of claim 12, wherein the atleast one oxide of metal B is selected from oxide of Ti, Zn, Sn, and W.15. A coated substrate prepared by a process comprising: (a) preparing asubstrate for deposition with a coating, wherein the coating comprisescrystallized particles of an oxide of a metal A having photocatalyticproperties, a mineral binder comprising at least one oxide of a metal Bhaving photocatalytic properties, optionally at least one oxide of ametal M devoid of photocatalytic properties, and optionally at least onesilicon oxide; (b) depositing the coating onto a surface of thesubstrate, by depositing the coating from liquid-phase dispersionscontaining oxides of the metals A and B, optionally the oxide of metal Mand optionally the silicon oxide, in a relative proportion by weight ofthe metals and Si given by A/(B+M+Si), the relative proportion rangingfrom 60/40 to 40/60; and (c) allowing the coating to set, to provide thecoated substrate, wherein the coating comprises crystallized particlesof the oxide of metal A having photocatalytic properties incorporatedinto the coating using an at least partially crystallized mineral bindercomprising the oxide of metal B also having photocatalytic properties inthe crystallized state, optionally at least one oxide of a metal Mdevoid of photocatalytic properties and optionally a silicon oxide,wherein the coating has a porosity of between 45% and 65% and coherencedomains having a size of about 5 nm to about 20 nm.
 16. The coatedsubstrate of claim 15, wherein the coating comprises silicon oxide. 17.The coated substrate of claim 15, wherein the at least one oxide ofmetal B is selected from oxide of Ti, Zn, Sn, and W.
 18. A coatedsubstrate prepared by a process comprising: (a) preparing a substratefor deposition with a coating, wherein the coating comprisescrystallized particles of an oxide of a metal A having photocatalyticproperties, a mineral binder comprising at least one oxide of a metal Bhaving photocatalytic properties, optionally at least one oxide of ametal M devoid of photocatalytic properties, and optionally at least onesilicon oxide; depositing the coating onto a surface of the substrate,by depositing the coating from liquid-phase dispersions containingoxides of the metals A and B, optionally the oxide of metal M andoptionally the silicon oxide, in a relative proportion by weight of themetals and Si given by A/(B+M+Si), the relative proportion ranging from60/40 to 40/60; and (c) allowing the coating to set, to provide thecoated substrate, wherein the coating comprises crystallized particlesof the oxide of metal A having photocatalytic properties incorporatedinto the coating using an at least partially crystallized mineral bindercomprising the oxide of metal B also having photocatalytic properties inthe crystallized state, optionally at least one oxide of a metal Mdevoid of photocatalytic properties, and optionally a silicon oxide,wherein the coating comprises a mineral binder comprising partiallycrystallized TiO₂ and crystallized TiO₂ particles essentially in anataseform, has a porosity, calculated from the refractive index, of between45% and 65%, and is present in an amount of between 15 and 45 μg/cm².19. The coated substrate of claim 18, wherein the coating comprisessilicon oxide.
 20. The coated substrate of claim 18, wherein the atleast one oxide of metal B is selected from oxide of Ti, Zn, Sn, and W.